Image information recorder having a resolution density transformation device

ABSTRACT

An image information recorder, including a printer capable of continuous printing in the direction of scanning lines, for reproducing binary input data of an image of a resolution density of 400 dpi (dots per inch) in a form of a visual image of the resolution density of 600 dpi. The input data of 400 dpi is transformed into multi-value image data of 600 dpi in accordance with two-dimensional filtering, which is converted into an analog image signal. The analog image signal is smoothed by a low pass filter, and then the smoothed analog image signal is binarized by a constant threshold signal, whereby an output binary image data of 600 dpi is produced.

This is a continuation of our U.S. application Ser. No. 07/294,677,filed Jan. 9, 1989, now U.S. Pat. No. 5,010,497, issued Apr. 23, 1991.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to an image information recorder, and moreparticularly, to an image information recorder capable of reproducingimage signals in the form of a visual image which have a resolutiondensity different from that of the recorder.

2. Description of the related art

Signals (image signals) of image information can be reproduced in theform of a visual image by an image information recorder, such as aprinter, which repeatedly carries out horizontal scanning (primaryscanning) and vertical scanning (secondary scanning) in response to theimage signals. The horizontal or primary one of the aforesaid scanningsis usually carried out along a so called "scanning" line.

The scanning line can be considered as being formed by a great manysmall dots, which continuously, or at least substantially continuously,exist on a line and each have a certain information, such as black orwhite, called a gray level. The image signals as mentioned above areprovided dot by dot in response to the gray level of the respectivedots.

The distance between two adjacent scanning lines is usually made equalto the diameter of a dot, so that dots are regularly arranged in amatrix (dot matrix). Therefore, any two-dimensional image can bereproduced by the collection of dots in a dot matrix which have therespective gray level according to the image. Such collection of dotswill hereinafter be called a dot pattern.

The quality, i.e., the resolution, of an image reproduced on the basisof the image signals is determined by the number of dots per unit lengthof a scanning line This number of dots is called a resolution density,which is usually represented by the number of dots per inch of ascanning line (abbreviated as dpi, hereinafter).

Presently, various kinds of office automation equipment, e.g.,computers, facsimile machines and so on, are used which are oftencoupled to image information recorders. In those cases, the resolutiondensity of image signals applied to an image information recorder mustbe equal to that of the recorder or else, the image information carriedby image signals can not be reproduced exactly. Therefore, in the casewhere it is necessary to have devices having the different resolutiondensities are coupled with each other, transformation of resolutiondensity is required between both devices.

For this purpose, various techniques of resolution densitytransformation have been developed. Briefly, resolution densitytransformation involves a dot pattern of a certain resolution densitybeing transformed to a dot pattern of a different resolution density. Inother words, when respective co-ordinate systems are applied to the dotmatrix of original image information and that of an image information tobe reproduced, the position and gray level of every dot in theco-ordinates of the image information to be transformed are determinedon the basis of the position and gray level of a dot or dots in theco-ordinates of the original image information.

The gray level of every dot in the transformed co-ordinates isdetermined by a calculation in accordance with a predetermined algorithmon the basis of the gray level of a dot or dots on the originalco-ordinates. During this calculation, there occur cases where some dotsin the original co-ordinates are sometimes omitted in the transformedco-ordinates or undesired dots are sometimes added in the transformedco-ordinates, with the result that blurring or fading appears in thevisual image reproduced.

The aforesaid disadvantage is caused for the following reason. The graylevel of a dot in the transformed co-ordinates, as the result of thecalculation in accordance with a predetermined algorithm, is of amulti-value. Such a gray level signal of the multi-value is comparedwith a certain threshold value to be converted into a binary signalsuited for reproduction by an image information recorder.

In this case, if the value of the threshold signal is always fixed,there occurs the omission of necessary dots or the addition of undesireddots, as mentioned above, according to the figure of the dot patternincluded in an original image information, e g., an oblique line, aright-angled pattern, and so on. In the case of an oblique line, forexample, the edges of a reproduced line are not become smoothed, but maybe indented because of the inappropriate selection of the value of athreshold signal. Also, the corner of a right-angled pattern may berounded

As one of the techniques for improving the disadvantage as mentionedabove, there has been proposed a resolution density transformationtechnique as disclosed in the laid-open Japanese patent applicationJP-A-62/73865(1987).

According to this prior art technique, the value of a threshold signalis varied in accordance with the gray level distribution in an originaldot pattern, which contributes to the calculation of the gray level of adot in the transformed co-ordinates. For example, when the gray level ofdots in the corner portion of a right-angled pattern is determined, athreshold signal is changed to a smaller value than usual. If,therefore, the dot pattern of an original image information is socomplicated that the figure as mentioned above is included very much,the threshold value must be changed often as much.

However, variance of the value of a threshold signal so often inaccordance with the gray level distribution in an original dot patternresults in complications, and such variance is not practical, especiallyin an image information having a complicated dot pattern.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image informationrecorder capable of reproducing image signals in the form of a visualimage with less distortion by means of a simple resolution densitytransformation, even if the image signals have the resolution densitydifferent from that of the recorder.

A feature of the present invention resides in that a signal representingthe calculated gray levels of dots in a density-transformed imageinformation is converted into an analog signal and then subjected to alow pass filter, and thereafter the analog signal is binarized by usinga constant threshold value.

According to a present invention, the high quality reproduction of imageinformation can be realized by an apparatus with simple structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the overall configuration of an imageinformation recorder according to an embodiment of the presentinvention;

FIGS. 2a to 2g are drawings for explaining a timing control unitemployed in the image information recorder of FIG. 1, in which there areshown a circuit arrangement of the unit and the waveforms of variousparts thereof;

FIG. 3 schematically shows the construction of an original dotextraction unit and a gray level calculation unit employed in the imageinformation recorder of FIG. 1;

FIGS. 4a to 4c and FIGS. 5a to 5d are drawings for explaining theoperation of the original dot extraction unit of FIG. 3;

FIG. 6 schematically shows an example of a circuit arrangement of a lowpass filter employed in the image information recorder of FIG. 1;

FIG. 7 schematically shows an example of a circuit arrangement of abinarizing unit employed in the image information recorder of FIG. 1;

FIG. 8 is a drawing for explaining the operational principle of atwo-dimensional filter included in the gray level calculation unit shownin FIG. 3; this figure will also be used to explain the principle ofresolution density transformation;

FIG. 9 shows an example of the dot pattern of a part of an originalimage information which is utilized for explaining the operation of theimage information recorder of FIG. 1;

FIGS. 10a to 10d and FIGS. 11a to 11e are drawings for explaining theoperation of the image information recorder shown in FIG. 1, which showthe waveforms of signals in various steps of the operation; and

FIGS. 12a and 12b comparatively show the dot patterns which aretransformed from the dot pattern of FIG. 9 according to the presentinvention and the prior art, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In an embodiment described below, there is discussed as an example animage information recorder which inputs an image information data of aresolution density of 400 dpi and reproduces such data in the form of avisual image by a printer having a resolution density of 600 dpi. Theimage information data may be output data from an electronic dataprocessing system or a facsimile data transmitted through acommunication line.

The printer as mentioned above can be a laser beam printer usually usedin an electronic data processing system or a thermal printer as used ina facsimile machine. The present invention is also applicable toprinters of the type in which substantially continuous printing can beaccomplished in the direction of primary scanning

For convenience in the following description, before a detaileddescription of the embodiment, the brief explanation will be given ofthe resolution density transformation with reference to FIG. 8.

FIG. 8 shows a part of a dot matrix applied to obtain image informationdata of a two-dimensional image, in which solid lines indicate the dotmatrix of original image information of a resolution density of 400 dpiand broken lines indicate the dot matrix of the image information whichis subjected to resolution density transformation from 400 dpi to 600dpi. In the original dot matrix, rows a₁ to a₄ are defined in thedirection of primary scanning, and columns b₁ to b₆ are defined indirection of the secondary scanning. Similarly, in the transformed dotmatrix, rows c₁ to c₆ are defined in the direction of the primaryscanning, and columns d₁ to d₉ are defined in the direction of thesecondary scanning.

Resolution density transformation means that a dot pattern in theoriginal dot matrix is transformed to a dot pattern in the transformeddot matrix. Namely, the gray levels of the dots (c₁, d₁), (c₁, d₂), . .. , (c₆, d₈), (c₆, d₉) in the transformed dot matrix are determined by acalculation in accordance with a certain algorithm on the basis of thegray levels of a dot or dots which are extracted from the dots (a₁, b₁),(a₁, b₂), . . . , (a₄, b₅), (a₄, b₆) in the original dot matrix. The dotor dots to be extracted are also determined by the certain algorithm.

Referring now to FIG. 1, the overall configuration of the imageinformation recorder of the embodiment will be described as follows.There is shown in the figure an example in which a laser beam printerhaving a resolution density of 600 dpi is used.

As shown in the figure, switchover unit 2 receives as input data animage information signal of a resolution density of 400 dpi, as alreadydescribed, and outputs it to original dot extraction unit 3, or directlyto laser beam printer 7, in accordance with particular circumstances. Ifa input data has the resolution density of 600 dpi, which is equal tothat of the printer 7, the switchover unit 2 will output the datadirectly to the printer 7 because no resolution density transformationis required. Otherwise, the switchover unit 2 outputs the received datato the original dot extraction unit 3 for processing of the resolutiondensity transformation.

The original dot extraction unit 3 receives the image information datafrom the switchover unit 2 and selects a dot or dots in the originalimage information on the basis of which the gray level of a dot in thetransformed image information is to be calculated. The operation in theunit 3 is carried out under the control of various timing controlsignals, including a synchronizing signal BD for primary scanning and aclock signal BC from the printer 7, which will be described in detaillater.

The gray level signals of the original dot or dots extracted in the unit3 are coupled to gray level calculation unit 4, in which the gray levelof a dot in the transformed image information is calculated inaccordance with a predetermined algorithm on the basis of the graylevels of the original dot or dots selected in the unit 3. As will bedescribed in detail later, a two-dimensional filter is utilized as thepredetermined algorithm in this embodiment. The calculating operation inthis unit 4 is also carried out under control of the synchronizingsignal BD and the clock signal BC from the printer 7. The calculatedgray level, which is of a digital multi-value, is converted into theanalog signal. Further, it is to be noted that the output signal of theunit 4 is an analog signal which changes stepwise.

The analog image information signal outputted from the unit 4 is coupledto low pass filter unit 5, in which the stepwise changing analog signalis smoothed. The smoothed signal outputted from the filter 5 is comparedwith a threshold signal of a predetermined constant value in binarizingunit 6 so that a binary signal of the transformed image information canbe obtained. The binary signal is coupled to the laser beam printer 7,in which the original image information is reproduced with thetransformed resolution density.

Timing control unit 8 receives the synchronizing signal BD and the clocksignal BC from the printer 7 and converts such signals into timingcontrol signals AD and AC for controlling the original dot extractionunit 3. Further, the signals BD and BC of the printer 7 are directlyapplied to both the original dot extraction unit 3 and the gray levelcalculation unit 4.

In the following, the timing control signals AD and AC and thepreparation thereof will be described, with reference to FIGS. 2a and2g.

As already described, the transformed image information data of 600 dpimust be obtained on the basis of the original image information data of400 dpi. In other words, data for three dots in the transformed imageinformation have to be obtained from data for two dots in the originalimage information. Therefore, it is necessary to provide the timingcontrol signals AD and AC, a frequencies of which have the ratio of 2:3with respect to the frequencies of the synchronizing signal BD and theclock signal BC of the printer 7, respectively.

The timing control signals AD and AC having the above mentionedfrequencies are generated by the timing control unit 8, which includestwo sets of the circuit arrangement shown in FIG. 2a. In each setthereof, there is provided counter 81 with preset terminals A, B, C, D,which are selectively coupled to a control voltage source V_(cc) topreset an initial value in the counter 81. In this case, the binary code"1101" is set, as shown in the figure. The initial value is set forevery time of application of a signal to a load terminal LO as describedbelow The

counter 81 further has output terminals Q_(A), Q_(B), Q_(C), Q_(D),carry output terminal CA and input terminal IN.

There are further provided two inverters 82, 83 and two NAND gates 84,85. Input terminals of the inverters 82, 83 are both coupled to theterminal IN of the counter 81. Two input terminals of the NAND gate 84are coupled to the terminal CA of the counter 81 and an output terminalof the inverter 82, respectively. An output signal of the NAND gate 84is applied to the terminal LO of the counter 81. The terminal Q_(C) ofthe third significant digit of the output terminals of the counter 81and an output terminal of the inverter 83 are coupled to two inputterminals of the NAND gate 85, respectively. The output signal of thetiming control unit 8 is derived from an output terminal of the NANDgate 85.

The circuit arrangement constructed as described above operates asfollows. The counter 81 counts up the input signal, as shown in FIG. 2b,applied to the terminal IN. As a result, the states of the terminalsQ_(C) and Q_(D) of the third and the least significant digits of theoutput terminals change as shown in FIGS. 2d and 2c, respectively,because the counter 81 repeatedly assumes three states of "1101", "1110"and "1111". Namely, the state of the counter 81 returns to "1101" from"1111" every third input signal applied to the terminal IN. Upon thestate change from "1111" to "1101", the carry output signal as shown inFIG. 2e appears at the terminal CA of the counter 81.

Because of occurrence of the carry output signal and the third inputsignal, the NAND gate 84 produces the load signal as shown in FIG. 2f,which is applied to the terminal LO of the counter 81. When the loadsignal occurs, the initial value of "1101" is set in the counter 81again. As the result of the operation as mentioned above, the outputsignal as shown in FIG. 2g can be obtained from the NAND gate 85. Asapparent from the comparison of FIGS. 2b and 2g, there can be obtainedthe output signal, the frequency of which has the ratio of 2:3 to thatof the input signal

The timing control unit 8 has two sets of the circuit as mentionedabove, one of which is used to produce the timing control signal AD asshown in FIG. 4b on the basis of the synchronizing signal BD as shown inFIG. 4a, and the other of which is used to produce the timing controlsignal AC as shown in FIG. 5c on the basis of the clock signal BC asshown in FIG. 5b. In the case of this embodiment, since thesynchronizing signal BD for the primary scanning has the repetitionperiod of about 0.5 msec and the clock signal BC has a repetition periodof about 50 nsec, the relation of the repetition period between thesignals BD and BC is as shown in FIGS. 5a and 5b.

Referring next to FIGS. 3, 4a to 4c and 5a to 5d, the details of theoriginal dot extraction unit 3 and the gray level calculation unit 4will be explained. The original dot extraction unit 3 comprises firstselector 31, three line memories 32 to 34, second selector 35 and P₁,P₂, Q₁, Q₂ registers generally denoted by reference numeral 36. The graylevel calculation unit 4 comprises two-dimensional filter 41, addresscontroller 42, latch 43 and digital to analog (D/A) converter 44.

The first selector 31 responds to the timing control signal AD as shownin FIG. 4b to input the data of the original image information sent fromthe switchover unit 2 into the line memories 32 to 34 successively Therespective line memories 32 to 34 have the same number of bits as thatof the dots in one scanning line.

As shown in FIG. 4c, therefore, the data for the row a₁ is stored in theline memory 32 in response to the first one of pulses of the timingcontrol signal AD, and the data for the row a₂ is stored in the linememory 33 in response to the next pulse of the signal AD. The data forthe row a₃ is stored in the line memory 34 in response to the thirdpulse of the signal AD. When the fourth pulse of the signal AD occurs,the data a₁ is put out from the line memory 32 and the data for the rowa₄ is newly stored therein.

As described above, the contents stored in the line memories 32 to 34are replaced successively in response to every pulse of the timingcontrol signal AD. Further, storage of the data for one scanning line iscarried out dot by dot (bit by bit) in synchronism with the timingcontrol signal AC as shown in FIG. 5c.

The second selector 35 responds to the synchronizing signal BD for theprimary scanning of the printer 7 and successively designates one of theline memories 32 to 34. Further, the selector 35 transfers the data ofthe designated one of the line memories 32 to 34 to the P₁, P₂ or Q₁, Q₂registers 36 in response to the clock signal BC of the printer 7.Further, the respective one of the P₁, P₂, Q₁ and Q₂ registers 36 iscomposed of a one-bit register. Therefore, the transfer of data from theline memories 32 to 34 to the one-bit registers 36 is as follows.

Assuming, for example, that the rows a₁ and a₂ are designated, the graylevels of the dots (a₁, b₁), (a₁, b₂) and (a₂, b₁), (a₂, b₂) are atfirst stored in the P₁, P₂ and Q₁, Q₂ registers 36 in response to firsttwo pulses of the clock signal BC, respectively. The contents of thoseone-bit registers change to the gray levels of the dots (a₁, b₂), (a₁,b₃) and (a₂, b₂), (a₂, b₃), when the next pulse of the clock signal BCoccurs. When a fourth pulse of the clock signal BC occurs, they furtherchange to the gray levels of the dots (a₁, b₃). (a₁, b₄) and (a₂, b₃),(a₂, b₄). In this manner, the contents of the four one-bit registers 36are successively changed in response to the synchronizing signal BD andthe clock signal BC of the printer 7.

The four one-bit registers 36 are coupled to the two dimensional filter41. In the filter 41, the gray levels of dots in the transformed imageinformation are calculated in accordance with a predetermined algorithmon the basis of the data stored in one or more than one of the fourone-bit registers 36. Concerning the algorithm to be executed by thefilter 41, a typical one of examples thereof will be described in detaillater.

The two-dimensional filter 41 is constructed by a gate circuitarrangement wired so as to realize the calculation according to thepredetermined algorithm or a microprocessor programmed so as to executethe predetermined algorithm. The data to be used for the calculation inthe filter 41 is selected from among the contents of the four one-bitregisters 36 in accordance with an address control signal suppliedthereto, which is produced by address controller 42 on the basis of thesynchronizing signal BD and the clock signal BC.

As is well known, an address controller of this kind is constructed bythe combination of a counter for counting any input pulse, such as thesynchronizing signal BD and the clock signal BC in this embodiment, anda decoder coupled thereto for decoding the content of the counter toproduce control signals. The address controller 42 is provided with twosets of counters and decoders constructed as described above.

One of the counters counts the synchronizing signal BD and one of thedecoders decodes the content of the one counter to produce a firstcontrol signal for designating the rows which include dots to be usedfor the calculation in the filter 41. The other counter counts the clocksignal BC and the other decoder decodes the content of the other counterto produce a second control signal for designating the columns whichinclude dots to be used for the calculation in the filter 41.

The address control signal produced by the address controller 42 iscomposed of the first and second control signals as mentioned above. Aswill be understood from the explanation described in detail later, thegray level calculation to be executed in the filter 41 is composed ofthe repetition of the same type of calculation, in which dots in theoriginal image information used for the calculation are regularlyshifted in a sequential manner. Therefore, the aforesaid decoders can beeasily constructed by gate circuits wired in accordance with thealgorithm of the gray level calculation to be executed in the filter 41.

Further, it is to be noted that, as will be apparent later, the graylevel signals of dots in the transformed image information as the resultof the above mentioned calculation are signals which take one of themulti-values, e.g., four values in this embodiment, although the datastored in the respective registers 36 is the binary data. Therefore,three bits are necessary for an output signal of the two-dimensionalfilter 41, whereby the four values of the signal level can berepresented.

The gray level signal calculated by the filter 41 is latched by thelatch 43 in response to the clock signal BC. Then, the latched data istransferred to the D/A converter 44 in synchronism with the clock signalBC and converted into the analog signal therein. The converted analogsignal as the output of the gray level calculation unit 4 is coupled tothe low pass filter unit 5.

The analog signal supplied from the gray level calculation unit 4 issmoothed by the low pass filter unit 5. In this respect, the followingshould be noted. The analog signal produced by the unit 4 is such thatthe signal level changes only at the boundary of two adjacent dotsnamely the analog signal from the unit 4 changes stepwise with respectto dots along a scanning line. On the contrary, the analog signalsmoothed by the unit 5 changes continuously with respect to dots along ascanning line. This means that the boundaries between adjacent dots areremoved in the gray level signal for one scanning line.

As the low pass filter unit 5, there can be employed a known circuitarrangement, an example of which is shown in FIG. 6. It comprisesamplifier 51, resistors 52, 53 and capacitors 54, 55. The analog signalfrom the gray level calculation unit 4 is applied to one end of theresistor 52, and the smoothed analog signal can be derived from anoutput terminal of the amplifier 51. The time constant, which isdetermined by the resistor 52 and the capacitor 54, can be selectedarbitrarily, but preferably is set at the value equal to the repetitionperiod of the pulses of the clock signal BC. Since the low pass filterconstructed as shown in the figure is already known, further explanationwill be omitted here

The analog signal smoothed by the unit 5 is coupled to the binarizingunit 6 and compared therein with a threshold signal of a predeterminedconstant value, whereby the smoothed analog signal can be converted bythe unit 6 into a binary signal suited for the printer 7. The gray levelof the binary signal obtained by the unit 6 does not always changeaccurately at the front edge of corresponding dots. For example, thegray level of a certain dot does not change from the front edge of thedot, but may first change at the center portion of the dot. However,even if the gray level changes at the center portion of a dot, noproblem occurs since the printer 7 can carry out substantiallycontinuous printing in the direction of the scanning line.

The aforesaid phenomenon is caused by the nature of the smoothed analogsignal as described above. Further, the amount of difference between thefront edge of a dot and a point at which the gray level of the dot firstchanges depends on the value of the threshold signal. This, however,occurs equally over the whole area of the original image information.Therefore, distortion is never caused in the image informationreproduced after the resolution density transformation. Further, ifnecessary, the aforesaid difference amount can be easily compensated tobe removed by a constant amount depending on the time constant of thelow pass filter unit 5 and the value of a threshold signal in thebinarizing unit 6.

Typical of examples of the circuit arrangement of the binarizing unit 6is the one shown in FIG. 7. It comprises amplifier 61, constantresistors 62, 63 and variable resistor 64. The value of the thresholdsignal can be determined by the resistance ratio of the constantresistor 63 and the variable resistor 64. The smoothed analog signalfrom the low pass filter unit 5 is applied to one (-) of the inputterminals of the amplifier 61, and the binarized signal can be obtainedfrom an output terminal thereof. Although various circuit arrangementsfor the binarizing unit are also known, the further description thereofwill be omitted here.

In the following, the description will be made of an example of thealgorithm for the gray level calculation. Although the variousalgorithms for that purpose are known already known, the two-dimensionalfilter is used in this embodiment.

In FIG. 8, as already described, the solid lines define the dots interms of 400 dpi of the original image information, and the broken linesdefine the dots in terms of 600 dpi of the transformed imageinformation. For the convenience of the following description, theformer dots will be called the original dots and the latter ones thetransformed dots. In the resolution density transformation, the graylevel of a transformed dot is determined on the basis of the calculationusing the gray levels of one or some of the original dots.

Attention is first called to the transformed row c₁. It can be seen inthe figure that the gray level of the transformed dot (c₁, d₁) can bedetermined on the basis of that of the original dot (a₁, b₁) only.Since, however, the transformed dot (c₁, d₂) extends to the two originaldots (a₁, b₁) and (a₁, b₂), the gray levels of the two original dots arenecessary to be taken into consideration in order to determine that ofthe transformed dot. The gray level of the transformed dot (c₁, d₂) canbe determined, for example, by the average of the gray levels of the twooriginal dots (a₁, b₁) and (a₁, b₂). Concerning the transformed dot (c₁,d₃), the gray level thereof can be determined again on the basis of thatof only one original dot, i.e., the dot (a₁, b₂).

As seen in FIG. 8, in the transformed row c₁, the same relation of thearrangement of the transformed dots to that of the original dots, asmentioned above, appears repeatedly and regularly every threetransformed dots. Therefore, the gray levels of the dots in the row c₁can be calculated by a set of the following formulas, in which the graylevel of a dot (a_(i), b_(j)) or (c_(i), d_(j)) is indicated by G(a_(i),b_(j)) or G(c_(i), d_(j)). ##EQU1## In the above formulas, k assumes 1,2, . . . , n, where n is the number of dots per scanning line.

Also in the transformed rows c₃, c₄, c₆, as seen in the figure, the graylevel of transformed dots can be determined in the similar manner to theabove. In the following, the formulas concerning the transformed row c₃will be exampled. ##EQU2##

As apparent from the comparison of the formulas (1) and (2), there isonly a difference in the original rows to be used in the gray levelcalculation for the transformed rows. Namely, the calculation of theformula (1) for the dots in the transformed row c₁ is carried out on thebasis of the gray levels of the dots in the original row a₁, whereas thecalculation of the formula (2) for the dots in the transformed row c₃ iscarried out on the basis of the gray levels of the dots in the originalrow a₂.

In the following description, therefore, the gray level calculation ofthis type will be identified by the formula (1) only. Further, it willbe understood that what one of the original rows is to be used in thecalculation for a transformed row is automatically determined, only ifthe the ratio, such as 2:3 in this embodiment, of the resolution densitytransformation is known.

The calculation for the transformed rows c₂ and c₅ is different fromthat for the aforesaid transformed rows, because, as apparent in FIG. 8,the transformed rows c₂, c₅ extend over the two adjacent original rows,i.e., the rows a₁ and a₂, and the rows a₃ and a₄, respectively.

Assuming, for example, that the transformed row c₂ is noted, the graylevel of the transformed dot (c₂, d₁) can be determined as the averagevalue of the gray levels of the two original dots (a₁, b₁) and (a₂, b₁).The gray level of the transformed dot (c₂, d₂) can be obtained by takingthe average value of the four original dots (a₁, b₁), (a₁, b₂), (a₂, b₁)and (a₂, b₂). Further, the gray level of the transformed dot (c₂, d₃)can be obtained again as the average value of the two original dots,i.e., the dots (a₁, b₂) and (a₂, b₂).

As seen in FIG. 8, in the transformed row c₂, the same relation of thearrangement of the transformed dots to that of the original dots appearsrepeatedly and regularly every three transformed dots. Therefore, thegray levels of the dots in the row c₂ can be calculated by a set of thefollowing formulas. ##EQU3##

In the similar manner, the calculation for the transformed row c₅ can becarried out, as follows. ##EQU4##

As apparent from the comparison of the above formulas (3) and (4), thereis only a difference in the original rows to be used in the gray levelcalculation for the transformed rows. Namely, the calculation of theformula (3) for the dots in the transformed row c₂ is carried out on thebasis of the gray levels of the dots in the original rows a₁ and a₂,whereas the calculation of the formula (4) for the dots in thetransformed row c₅ is carried out on the basis of the gray levels of thedots in the original rows a₃ and a₄.

In the following description, therefore, the gray level calculation ofthis type will be identified by the formula (3) only. Further, in thesimilar manner to the description concerning the gray level calculationfor the rows c₁, c₃, c₄ and c₆, it will be understood that what ones ofthe original rows are to be used in the calculation for the rows c₂ andc₅ is automatically determined, only if the ratio, i.e., 2:3 in thisembodiment, of the resolution density transformation is known.

Further, as apparent from FIG. 8, the same relation between thearrangement of the original dots and that of the transformed dotsappears repeatedly and regularly. Therefore, what one of the threeformulas in the formula (1) or (3) should be used is determined by onlycounting pulses of the clock signal BC. In the similar manner, which oneof the set of the formula (1) and the set of the formula (3) should beused can be determined by only counting pulses of the synchronizingsignal BD.

In the following, there will be explained the operation of the imageinformation recorder as shown in FIG. 1. For this purpose, there istaken into consideration an example of an original image information ofthe resolution density of 400 dpi. The dot pattern of a part of theimage information is shown in FIG. 9.

In FIG. 9, hatched portions indicate black dots in the original imageinformation and non-hatched portions indicate white dots therein. If ablack dot is represented by a binary signal of level "1" and a white dotby that of level "0", the image signal for the row a₁ in the dot patternshown has the profile as shown in FIG. 10a with respect to the columnsb₁ to b₆. This also corresponds to the content of one of the linememories 32 to 34 in the original dot extraction unit 3.

The original dot extraction unit 3 selects the dots, which are necessaryfor the calculation of a transformed dot. The gray level calculationunit 4 calculates the gray level of the transformed dot in accordancewith the formula (1) on the basis of the gray levels of the originaldots selected in the unit 3. The calculation result is subjected to theD/A conversion and the analog signal as shown in FIG. 10b is produced bythe gray level calculation unit 4.

The output signal of the unit 4 is smoothed by the low pass filter unit5 and changed to the signal as shown in FIG. 10c. The smoothed signal iscompared in the binarizing unit 6 with the threshold signal as shown bya broken line in FIG. 10c, with the result that the binary signal asshown in FIG. 10d can be obtained.

Referring next to FIGS. 11a to 11e, the calculation concerning thetransformed row c₂ will be explained. For the calculation for the rowc₂, the two original rows a₁ and a₂ as shown in FIGS. 11a and 11b arenecessary to be taken into consideration. The original dot extractionunit 3 selects original dots necessary for the calculation of every dotof the row c₂ from these two rows a₁ and a₂.

The gray level calculation unit 4 calculates the gray level of thetransformed dot in accordance with the formula (3) on the basis of thegray levels of the selected original dots. The calculation result isconverted into the analog signal, as shown in FIG. 11c. The analogsignal is smoothed by the low pass filter unit 5 to be changed as shownin FIG. 11d. Then, the smoothed signal is binarized in the binarizingunit 6 by being compared with the threshold signal as shown by a brokenline in FIG. 11d, so that the binary signal as shown in FIG. 11e can beobtained.

If the calculating operation as mentioned above is completed withrespect to all the rows, the image information of 400 dpi can betransformed into the image information of 600 dpi. There is shown inFIG. 12a the result of the resolution density transformation of the dotpattern as shown in FIG. 9 from 400 dpi to 600 dpi. For the purpose ofthe comparison, there is shown in FIG. 12b the result of the resolutiondensity transformation from 400 dpi to 600 dpi according to the priorart described in the description of the related art.

In the prior art resolution density transformation, there can be seenthe occurrence of distortion in the portions shown by broken circles inFIG. 12b, though such complicated processing such that the value of athreshold signal is varied in accordance with the distribution of thedot pattern is carried out. On the other hand, according to theresolution density transformation of the present invention, considerablyexcellent reproduction of the original image information can be realizedwithout the complicated variation of the threshold as in the prior art.

I claim:
 1. An image information recorder comprising:printer means,capable of substantially continuous printing in a scanning linedirection, for reproducing discrete binary input data of an image havinga first resolution density supplied to said printer means as data of avisual image having a second resolution density in accordance with thediscrete binary input data, said discrete binary input datacorresponding to a first line of pixels having the first resolutiondensity and changing in state only at pixel borders within the firstline; resolution density transformation means for transforming thediscrete binary input data having the first resolution density todiscrete multi-value image data having the second resolution density inaccordance with a predetermined algorithm and converting the discretemulti-value image data into analog form to produce a continuous analogimage signal, said discrete multi-value image data corresponding to asecond line of pixels having the second resolution density and changingin state only at pixel borders within the second line, and saidcontinuous analog signal corresponding to the second line of pixels andchanging in state at positions other than the pixel borders of thesecond line; low pass filter means for smoothing the continuous analogimage signal produced by said resolution density transformation means;and binarizing means for comparing the continuous analog image signalsmoothed by said low pass filter means with a threshold value having apredetermined constant value to produce continuous binary imageinformation data to be supplied to said printer means, said continuousbinary image information data corresponding to said second line ofpixels and changing in state at positions other than the pixel bordersof the second line.
 2. An image information recorder according to claim1, further comprising switchover means for supplying the discrete binaryinput data to said printer means if a user determines that the first andthe second resolution densities are equal to each other, and forsupplying the discrete binary input data to said resolution densitytransformation means if a user determines that the first and the secondresolution densities are different from each other.
 3. An imageinformation recorder comprising:printer means, capable of substantiallycontinuous printing in a scanning line direction, for reproducingdiscrete binary input data of an image having a first resolution densitysupplied to said printer means as data of a visual image having a secondresolution density in accordance with the discrete binary input data,said discrete binary input data corresponding to a first line of pixelshaving the first resolution density and changing in state only at pixelborders within the first line; original dot extraction means forreceiving therein the discrete binary input data in response to apredetermined timing control signal generated on the basis of asynchronizing signal used for scanning and a clock signal from saidprinter means and for selecting dots from the discrete binary input datawhich contribute to a gray level calculation of a dot in the image to bereproduced in response to the synchronizing signal used for scanning andthe clock signal; gray level calculation means, responsive to thesynchronizing signal and the clock signal, for carrying out the graylevel calculation in accordance with a predetermined algorithm based ongray levels of the dots selected by said original dot extraction meansand for converting a result of the gray level calculation into analogform to produce a continuous analog image signal, said continuous analogimage signal corresponding to a second line of pixels and changing instate at positions other than the pixels borders of the second line; lowpass filter means for smoothing the continuous analog image signalproduced by said gray level calculation means; and binarizing means forcomparing the continuous analog image signal smoothed by said low passfilter means with a threshold signal of a predetermined constant valueto produce continuous binary input data to be supplied to said printermeans, said continuous binary input data corresponding to said secondline and changing in state at positions other than the pixel borders ofthe second line.
 4. An image information recorder according to claim 3,further comprising timing control means for producing the timing controlsignal by changing a frequency of the synchronizing signal or the clocksignal in accordance with a ratio of the first and the second resolutiondensities.